Basic SPC Control Chart

Mix.install([
  {:plotly_ex, "~> 0.3"},
  {:kino, "~> 0.18"}
])

Section

A Statistical Process Control (SPC) chart tracks a process over time with three horizontal reference lines: the mean (centre line), and upper/lower control limits (UCL/LCL), typically set at ±3σ.

layout.shapes draws horizontal lines across the full x range. Each shape entry is a map with type: "line", x0/x1 (or use "paper" coordinates), and y0/y1 set to the same value for a horizontal line.

alias Plotly.{Figure, Scatter}

:rand.seed(:exsss, {10, 20, 30})

n = 30
# Simulated process measurements with occasional shift
process = for i <- 1..n do
  base = (for _ <- 1..12, do: :rand.uniform()) |> Enum.sum() |> Kernel.-(6.0)
  if i > 20, do: base + 1.5, else: base
end

mean  = Enum.sum(process) / n
sigma = :math.sqrt(Enum.sum(Enum.map(process, fn v -> (v - mean) * (v - mean) end)) / n)
ucl   = mean + 3 * sigma
lcl   = mean - 3 * sigma

Figure.new()
|> Figure.add_trace(
  Scatter.new(
    x: Enum.to_list(1..n),
    y: process,
    mode: "lines+markers",
    name: "Measurements",
    marker: %{size: 6}
  )
)
|> Figure.update_layout(
  title: "Basic SPC Control Chart",
  shapes: [
    %{type: "line", x0: 1, x1: n, y0: mean, y1: mean,
      line: %{color: "green", dash: "solid", width: 2}},
    %{type: "line", x0: 1, x1: n, y0: ucl, y1: ucl,
      line: %{color: "red", dash: "dash", width: 1.5}},
    %{type: "line", x0: 1, x1: n, y0: lcl, y1: lcl,
      line: %{color: "red", dash: "dash", width: 1.5}}
  ],
  annotations: [
    %{x: n, y: mean, xanchor: "left", text: "Mean", showarrow: false},
    %{x: n, y: ucl,  xanchor: "left", text: "UCL (+3σ)", showarrow: false},
    %{x: n, y: lcl,  xanchor: "left", text: "LCL (−3σ)", showarrow: false}
  ]
)
|> Plotly.show()